1. Field of the Invention
The present invention relates to a mobile packet communication system for supporting mobile terminals in a network which is composed of a plurality of auxiliary networks interconnected by routers, and more particularly to a mobile packet communication system for transmitting packets of data that are encapsulated by a maximum of N headers of the-same format.
2. Description of the Related Art
In computer networks which support mobile terminals, it has been known to effect routing to a mobile terminal by defining an inherent address which represents the mobile terminal itself, rather than depending on the position thereof, and a positional address which varies depending on the position of the mobile terminal, and then determining an association between the inherent and positional addresses.
When mobile terminals are supported using the above process, no communications can be carried out unless a database giving an association between the inherent and positional addresses of the mobile terminals is properly maintained. One example of a packet format which is described in Document 1: Fumio Teraoka, "VIP: Protocol which provides host movement transparency", Computer Software, Vol. 10, No. 4 (1993), pp. 22-38, is shown in FIG. 1 of the accompanying drawings.
In FIG. 1, IP (Internet Protocol) addresses that are widely used in computer networks are employed as inherent and positional addresses.
In FIG. 1, the reference numeral 1000 pointing to a solid-line frame represents a packet header based on positional addresses, and the reference numerals 1001, 1002 represent positional addresses of a source and a destination in the packet header 1000 based on positional addresses. The reference numeral 1010 pointing to a broken-line frame represents a packet header based on inherent addresses, the reference numerals 1011, 1012 represent version information of inherent and positional addresses of a source and version information of inherent and positional addresses of a destination, the version information being indicated by time, and the reference numerals 1013, 1014 represent inherent addresses of a source and a destination.
The association between the inherent and positional addresses of respective mobile terminals is managed in all routers on paths of communications effected by the mobile terminals, as shown in FIG. 2 of the accompanying drawings.
In FIG. 2, the reference numerals 1100-1104 represent auxiliary networks Net-0-Net-5, the reference numerals 1105-1108 represent routers Router1-Router4, and the reference numerals 1110-1112 represent mobile terminals Host-1-Host-3.
In the example shown in FIG. 2, the mobile terminal Host-1 (1110) is communicating with the mobile terminals Host-2 (1111), Host-3 (1112), and a home network for the mobile terminals Host-1 (1110), Host-2 (1111), Host-3 (1112) is the auxiliary network Net-0 (1100). Therefore, the mobile terminals Host-1 (1110), Host-2 (1111), Host-3 (1112) have respective inherent addresses depending on the auxiliary network Net-0 (1100). Since the mobile terminals Host-1 (1110), Host-2 (1111), Host-3 (1112) are connected respectively to the auxiliary networks Net-0 (1100), Net-1 (1101), Net-4 (1104), the mobile terminals Host-1 (1110), Host-2 (1111), Host-3 (1112) have respective positional addresses depending on the respective auxiliary networks Net-0 (1100), Net-1 (1101), Net-4 (1104).
For communications between the mobile terminals Host-1 (1110), Host-2 (1111) existing on communication paths, address association information relative to the mobile terminals is maintained in the routers Router1 (1105), Router2 (1106). For communications between the mobile terminals Host-1 (1110), Host-3 (1112) existing on communication paths, address association information relative to the mobile terminals is maintained in the routers Router1 (1105), Router3 (1107), Router4 (1108). While the router Router3 (1107) is not directly involved in communications between the mobile terminals Host-1 (1110), Host-2 (1111), the router Router3 (1107) maintains address association information relative to the mobile terminal Host-2 (1111) because it can peek at packets transmitted between the mobile terminals Host-1 (1110), Host-2 (1111). The router Router1 (1105) which is connected to the home network Net-0 (1100) for the mobile terminals Host-1 (1110), Host-2 (1111), Host-3 (1112) necessarily maintains address association information of these mobile terminals Host-1 (1110), Host-2 (1111), Host-3 (1112). Each of the mobile terminals Host-1 (1110), Host-2 (1111), Host-3 (1112) maintains address association information of the mobile terminal which it is presently communicating with.
To keep consistent a database of address association information of the mobile terminals in the routers Router1 (1105)-Router4 (1108), each data packet which is transmitted contains the version information of inherent and positional addresses of a source and a destination as indicated by the packet format shown in FIG. 1. Each of the routers Router1 (1105)-Router4 (1108) compares the version information of inherent and positional addresses of a source and a destination contained in all packets which are received with corresponding version information of inherent and positional addresses entered in its own database. If the compared items of version information differ from each other, then each of the routers Router1 (1105)-Router4 (1108) recognizes that the inherent and positional addresses are inconsistent, and effects a control process to eliminate the inconsistency.
FIG. 3 of the accompanying drawings shows headers and transmission data separately. In FIG. 3, the reference numeral 3000 represents a header containing information of a present position, the reference numeral 3001 represents a header containing information that indicates a mobile terminal itself, and the reference numeral 3002 represents transmission data.
According to Document 1, referred to above, a new value VIPprot is defined to indicate that the transmission data is encapsulated by the two headers, i.e., as shown in FIG. 3, the header 300 containing information of a present position and the header 3001 containing information that indicates a mobile terminal itself. The new value VIPprot thus defined is inserted in a field (Protocol field) indicating a higher protocol of the header 3000.
A transmission system described in Document 1 will be described below.
According to the process disclosed in Document 1, routing to a mobile terminal in a computer network which support mobile terminals is effected by defining an inherent address which represents the mobile terminal itself, rather than depending on the position thereof, and a positional address which varies depending on the position of the mobile terminal, and then determining an association between the inherent and positional addresses. The process is based on the assumption that all the mobile terminals are capable of processing a packet having two addresses, i.e., an inherent address and a positional address. Under transient conditions, however, there may be situations in which mobile terminals are capable of processing packets having two such addresses and fixed terminals are capable of processing packets having only inherent addresses.
To cope with the above situation, Document 1 discloses a system for determining whether a terminal which a mobile terminal is to communicate with is a mobile terminal or a fixed terminal, transmitting packets having two addresses, i.e., an inherent address and a positional address, to the terminal if it is a mobile terminal, and transmitting packets having only inherent addresses to the terminal if it is a fixed terminal.
Such a packet transmitting system of a mobile terminal is shown in FIG. 4 of the accompanying drawings. As shown FIG. 4, the packet transmitting system has an input terminal 2100', an output terminal 2105', an inherent address header generator 2101', a mobile terminal/fixed terminal decision unit 2102', a positional address header generator 2103', and a switch 2104'.
For transmitting a packet, the inherent address header generator 2101' generates an inherent address header. At the same time, the inherent address header generator 2101' supplies a destination address to the mobile terminal/fixed terminal decision unit 2102'. The mobile terminal/fixed terminal decision unit 2102' determines whether the destination indicated by the destination address from the inherent address header generator 2101' indicates a mobile terminal or not.
If the destination is a mobile terminal, then the mobile terminal/fixed terminal decision unit 2102' indicates a positional address of the destination to the positional address header generator 2103', and controls the switch 2104' to output a positional address header generated by the positional address header generator 2103' to the output terminal 2105'.
If the destination is a mobile terminal, then the mobile terminal/fixed terminal decision unit 2102' controls the switch 2104' to output the inherent address header generated by the inherent address header generator 2101' to the output terminal 2105'.
Consequently, if the destination is a mobile terminal, then the packet transmission system transmits a packet based on two addresses, i.e., a positional address and an inherent address, and if the destination is a mobile terminal, then the packet transmission system transmits a packet based on only an inherent address.
In the conventional arrangement shown in FIGS. 1 and 2, each router compares the version information of inherent and positional addresses of a source and a destination contained in all packets which are passing with corresponding version information of inherent and positional addresses entered in its own database in order to effect a control process to eliminate any inconsistency. Therefore, the system is subjected to an undue burden. Furthermore, since there is a possibility that corresponding data of inherent and positional addresses of a certain terminal exist in many routers, the control process to eliminate any inconsistency requires an unduly large amount of communications. In addition, the data of a header is large because each packet contains version information for mapping inherent and positional addresses of a source and a destination.
In the conventional format shown in FIG. 3, when a new value VIPprot is defined which indicates that transmission data is encapsulated by the two headers, i.e., the header 300 containing information of a present position and the header 3001 containing information that indicates a mobile terminal itself, any existing network device which does not incorporate the new value VIPprot that is defined cannot process encapsulated transmission data, and hence compatibility with existing systems is greatly impaired. Specifically, when an existing network device which does not incorporate the new value VIPprot that is defined receives a packet of transmission data encapsulated with the field value of VIPprot, the network device cannot interpret the value of VIPprot, and will discard the packet.
In the packet transmission system shown in FIG. 4, each mobile terminal determines whether the destination of a packet is a mobile terminal or a fixed terminal, changes the format of the packet depending on the determined result, and then transmits the packet. It is desirable that any processing in mobile terminals be minimized in view of needs for smaller and lighter mobile terminals and the period of time for which batteries are connected to mobile terminals. The conventional packet transmission system cannot meet such demands because packet processing in mobile terminals places an increased burden on the mobile terminals.
Packets transmitted from mobile terminals to fixed terminals are required to use a format which is inconsistent with a protocol hierarchy which supports packet movement, for the reception of the packets at the fixed terminals. The use of such a format is disclosed in Document 2 given below:
Document 2: Fumio Teraoka, "Protocol Transition from IP to VIP", Japan Software Science Society, Collected articles of 9th Conference, pp. 505-508.